The present invention is directed to a free machining aluminum alloy containing bismuth as a free machining constituent thereo, or bismuth and tin as machining constituents and a method of use and, in particular, to a free machining aluminum alloy containing bismuth as a low melting point elemental discontinuity or bismuth and tin, each of which provides improved machining without loss of mechanical properties.
Free machining aluminum alloys are well known in the art. These alloys typically include free machining compounds such as lead-tin, indium-bismuth, and tin for improved machinability. In many of these alloys, these elements form low melting point compounds which readily melt or soften due to the friction heat created during machining. More specifically, at the point of contact between the machining tool and the material, softening and melting occurs. As a result of these changes, breakage occurs, chips are formed and material removal is enhanced.
When using free machining constituents having higher melting points than the eutectic-types noted above, material removal results from a different mechanism. At the point of contact with the machining tool, void formation occurs as a result of the different flow characteristics between the base aluminum and the high melting point constituent. This void formation then causes breakage and subsequent chip formation. Formation of the chips equates to material removal.
When using low melting point constituents, the flow characteristics of the constituents is similar to that of the aluminum base material. Consequently, void formation as a mechanism for material removal does not appear to be a plausible advantage.
One example of a free machining alloy is disclosed in U.S. Pat. No. 5,522,950 to Bartges et al. This patent discloses a substantially lead-free AA6XXX aluminum alloy which is substantially free of lead, bismuth, nickel, zirconium and cadmium. The free machining element in the Bartges et al. patent is tin only in amounts between about 1.01% and 1.5% by weight. In this patent, the tin could have a beneficial effect on material removal both from the standpoint of void formation, and as a relatively low melting point constituent (as compared to high melting point constituents) with respect to the aluminum base.
While the Bartges et al. patent provides improvements in free machining alloys by limiting the levels of lead and bismuth, the presence of tin adversely effects the alloysxe2x80x2 mechanical properties, particularly impact properties. In other words, adding tin only makes this alloy brittle and renders it unacceptable where impact properties in a particular application may be important.
Another drawback associated with free machining alloys containing tin is a lack of corrosion resistance in environments where hot brake fluid is present. Brake system components made from tin-containing free machining alloys exhibit excessive corrosion in the presence of brake fluid.
In light of the deficiencies described above for tin-containing free machining aluminum alloys, a need has developed to provide an improved free machining aluminum alloy which overcomes the prior art deficiencies now in existence.
In response to this need, the present invention, in one embodiment, provides a free machining aluminum alloy that utilizes effective amounts of bismuth as a free machining elemental constituent. In another embodiment, the aluminum alloy has effective amounts of bismuth and tin to greatly improve machinability.
Bismuth-containing aluminum alloys have been proposed as bearing materials as disclosed in U.S. Pat. No. 5,286,445 to Soji. Bismuth is added to these alloys to enhance self lubricity and these alloys are not generally used or adaptable as free machining alloys.
Machinability improvement via bismuth addition is achieved by either softening of the bismuth particles during local temperature rise during machining or by void formation due to deformation mismatch between bismuth and the aluminum matrix during machining. It is also possible that a combination of the two processes is at play during machining which gives rise to improvement in machinability.
Accordingly, it is a first object of the present invention to provide an improved free machining aluminum alloy.
Another object of the present invention is to provide an aluminum alloy having free machining constituents which do not deleteriously affect mechanical properties, particularly impact properties.
A still further object of the present invention is to provide a method of machining aluminum alloy articles using a bismuth-containing or bismuth and tin-containing aluminum alloy.
One other object of the present invention is to provide machined aluminum alloy products from the inventive methods.
Yet another object of the invention is the use of bismuth or bismuth and tin as a substitute free machining element or elements for other free machining constituents in free machining aluminum alloys.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention is an improvement over prior art free machining aluminum alloys. In contrast to existing free machining aluminum alloys, in one embodiment, the inventive alloy utilizes bismuth as a low melting point free machining elemental constituent. The bismuth is controlled so that it occupies between about 0.1% and about 3.0% by weight of the total composition.
When using bismuth alone, the bismuth is preferably uniformly dispersed throughout the alloy so that effective machining is achieved regardless of the orientation between a workpiece made of the inventive alloy and a machining tool.
In another embodiment, bismuth and tin are employed together as free machining constituents so as to total together the levels described for bismuth alone. When using these elements together, the amount of bismuth ranges between 0.1% and about 3.0% by weight, and the amount of tin ranges between about 0.1 and 1.5% by weight.
The free machining elemental constituents are believed to be applicable to aluminum alloys such as the AA1000 series, AA2000 series, AA3000 series, AA4000 series, AA5000 series, AA6000 series, and AA7000 series. More preferred classes of alloys include the AA2000, AA4000, and AA6000 series aluminum alloys.
More preferred weight percents for bismuth alone range between about 0.1% and 1.5%, between about 0.1% and 1.0% and between about 0.2% and 0.8%, respectively. More preferred weight percentage ranges for each of bismuth and tin when used together comprise between 0.1 and 1.3%, between 0.1 and 1.0%, and between 0.1 and 0.85%.
The invention also includes a machining process whereby an aluminum alloy article made from the inventive free machining alloy composition is machined to a desired shape. The invention also encompasses the machined article made by the inventive method.
Another aspect of the invention details a method of improving the impact properties of free machining aluminum alloys by providing a molten AA6000 series aluminum alloy and adjusting its composition by adding an amount of bismuth or bismuth and tin so that the final alloy composition has between about 0.1% and 3.0% by weight of the total of the added amount when bismuth alone is used and between 0.1 and 3.0% by weight of bismuth and between 0.1 and 1.5% by weight of tin, when combined together. The solidified alloy can then be subjected to machining with the machined article having no deleterious effects on mechanical properties, especially impact properties.